1. Field of the Invention
The present invention relates to a press pad for use in a laminating press for the production of laminate sheets, such as decorative laminates, laminated floorboards, and printed circuit boards, using low pressure and high pressure single daylight and multi-daylight presses.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
The purpose of a press pad is to compensate for density variations in the laminate sheet being pressed and thereby to ensure that an equal pressure is applied to all parts of the sheet. In addition, the press pad compensates for any unevenness in the surfaces of the platens of the press itself and any flexure or bowing of the platens when under pressure. Again, this assists in the production of a flat even density laminate. Thus, it is important for a press pad to be resilient and have a natural springiness to permit it to compensate for the aforementioned density variations and the surface unevenness of the press platens but also to allow it to relax after each pressing operation and recover its form to enable it to be used again. The capacity a press pad has to re-form itself after each pressing is an important characteristic to ensure a reasonable working life and to avoid unnecessary downtime of a press while press pads are replaced.
Typically, therefore, a conventional press pad is a densely woven combination of high temperature resistant non-asbestos yarns and metal wire. The metal wire is included to give good heat transmission through the pad to the laminate sheet. In contrast, the non-metal yarn is required to give the pad the springiness and resilience required to enable the pad to relax after each pressing operation. The relative proportion of the two types of material is a consideration when devising a press pad for a particular purpose. Usually a compromise must be reached between the heat transference and the resilience or springiness required in each case.
A conventional press pad is described in European Patent No. 0 735 949 A1. The pad comprises a woven fabric of heat resistant strands such as copper wires, wherein a substantial proportion of either the warp or the weft comprises a silicone elastomer. In practice, as shown in FIG. 1, the warp 1 usually comprises stranded or bunched brass or copper wire and the weft 2 usually comprises a silicone covered metal wire. In particular, a sheath 4 of silicone has been extruded over a stranded or bunched copper wire 3. As a result of the presence of the silicone 4, this press pad has great resilience and springiness, while the metal wires ensure that the press pad achieves good heat transference from the platens to the material being pressed.
Hitherto, the copper wire covered by the silicone sheath has been copper wire comprising seven individual strands of 0.2 mm diameter that have been stranded or bunched. Stranded wire comprises a wire wherein the strands are subjected to a positive and controlled twisting with one of the seven strands forming a core around which the other six strands are wrapped. Such a wire 5 is illustrated in FIG. 2, wherein a strand forming the central core 6 is shown surrounded by six strands 7 that have been twisted around it. If the strands 6, 7 each have a diameter of 0.2 mm, then it can be seen that the overall wire diameter d1 (see FIG. 1) is 0.6 mm. In contrast, bunched wire comprises a wire wherein the strands are twisted in a more random fashion without any of them having the center position. Such a bunched wire also has an overall diameter of approximately 0.6 mm if seven single strands of 0.2 mm are used. The degree of twist used in the stranded or bunched wire of both the warp 1 and the weft 2 is typically of the order of ‘15 mm lay’. This represents the length of finished wire required for a 360° twist in the twisted strands.
When coated with silicone, the outside diameter d2 of the silicone-covered weft 2 is typically 1.4 mm, thus making the wall thickness d3 of the silicone 0.4 mm. Typically, the woven press mats using such silicone-covered wire have an initial thickness T1 (see FIG. 1) of 2.5 mm but after a relatively short use they settle to a thickness of around 2.0 mm. This is because the warp wires are pushed into the silicone of the weft. In this state, a press pad may typically achieve 200,000 press cycles before it becomes spent. The pad wears out because, in use, the weave structure is eventually flattened to such an extent that the press pad is unable to relax after each pressing operation, and the pad loses its resilience and springiness.
Press pads are used in presses exerting an average specific pressure of around 35 kg per cm2 so that the total download on one square meter of press pad material is 350,000 kg. A typical press pad has around 550 weft insertions per meter of length and 900 warp threads per meter of width. This means that there are typically 550×900=495,000 crossover points per square meter of press pad, each being subjected, in use, to a download of around 0.707 kg during each compression cycle of the press. During use, at each crossover point the warp wire 1 fairly quickly cuts through the silicone coating 4 of the weft 2 and, before deformation of the crossing wires owing to the applied pressure, there are two wires 1, 3 each of 0.6 mm touching each other. This is shown schematically in FIG. 3 without any silicone being depicted. Over time, during continued use of the press pad, the two crossing wires 1, 3 become compressed into each other and their total thickness of 0.6 mm+0.6 mm=1.2 mm might reduce to around 0.8 mm. This is a typical final thickness of a press pad once it has been spent and it has ceased to act as a flexible compensation mat. The silicone has, by then, been pressed into the interstices between the wire mesh formed by the crossing wires 1, 3 and the wire crossover points support the total download.
The number of pressing cycles which has to occur before a conventional pad similar to that described above becomes worn out is dependent to a very large degree upon the nature of the laminated sheets being pressed. Decorative laminates have an inherent springiness and resilience so that during a pressing operation they also assist in providing the compensation required. However, laminated floorboards made from medium and high density fiberboard have very little natural springiness and it has been found that conventional press pads as described above wear out relatively quickly when used for pressing these types of laminates.
It is an object of the present invention to provide a press pad which will retain its springiness and compensation ability for a greater number of pressing cycles than a conventional press pad without any loss of heat transfer ability.